Manipulating internal recirculation flow rate on the biological process in wastewater treatment

Internal recirculation plays an important role in the different biological treatment areas of a wastewater treatment plant, because it has a great influence on the concentration of pollutants, especially nutrients. The most common uses of the internal recirculation flow rate are to keep it fixed or to manipulate it to obtain a fixed nitrate set-point in the last anoxic tank using control techniques. This work proposes a new control strategy for manipulating the internal recirculation flow rate by applying a fuzzy controller. The proposed controller takes into account the effects of the internal recirculation flow rate at the inlet of the biological treatment and in the denitrification and nitrification processes with the aim of reducing violations of legally established limits of nitrogen and ammonia and also reducing operational costs. The proposed fuzzy controller is tested by simulation with the internationally known benchmark simulation model no. 2. The default control strategy with a hierarchical dissolved oxygen control is used and the results are compared by keeping the recirculation flow fixed (as the default control strategy) and manipulating it with the proposed fuzzy controller . The results show reduction improvements of 68.78% of total nitrogen limit violations, 24.92% of ammonia limit violations and 3.79% of pumping energy costs.